Titanium dioxide is used as an opacifier and colorant in many industries, including the coatings, plastics, and paper industries. In general, the effectiveness of the pigment in such applications depends on how evenly the pigment can be dispersed in a coating, in plastic or in paper. For this reason, pigments are generally handled in the form of a finely divided powder. However, titanium dioxide powders are inherently dusty and frequently exhibit poor powder flow characteristics, especially during formulation, compounding, and manufacture of end-use products. While free-flowing powders with low dust properties can be obtained through known manufacturing practices, these powders usually exhibit reduced opacifying properties.
To this end, chemical methods of modification of titanium dioxide pigment surfaces have been developed to achieve the desired balance of pigment opacity and flow characteristics. For instance, it is known in the art that the wetting and dispersing properties of titanium dioxide pigments can be improved by exposure of a titanium dioxide intermediate (produced by either a sulfate or chloride process) to certain inorganic treatments through the deposition of inorganic metal oxide and/or metal hydroxide coatings on the surface of the titanium dioxide. Typically these treatments are accomplished by:
(1) dispersing the intermediate (or crude) material in an aqueous medium using a dispersing agent such as a polyphosphate,
(2) optionally wet milling the resulting slurry to achieve a certain desired particle size,
(3) precipitating one or more inorganic oxides such as silica or alumina onto the particle surfaces of the titanium dioxide slurry,
(4) recovering the inorganic oxide treated titanium dioxide pigment from the aqueous slurry by filtration,
(5) washing the filtered product to remove salts and impurities,
(6) drying the washed filtered product, and
(7) dry-milling the dried pigment using a fluid energy mill.
Typically the wet treatment deposition of inorganic oxides according to step (3) is accomplished—for pigments treated with more than one inorganic oxide—in a sequential fashion, one inorganic oxide at a time. However, it is also known to chemically treat titanium dioxide pigment intermediates with co-precipitated mixed inorganic oxides. Apart from reducing the total number of inorganic surface treatments to be performed, titanium dioxide pigments bearing co-precipitated mixed inorganic oxide treatments perform differently as compared to pigments wherein the same inorganic oxides are added sequentially.
A number of references describe or at least suggest titanium dioxide pigments including a co-precipitated, mixed inorganic oxide surface treatment. For example, U.S. Pat. No. 2,913,419 discloses a broad range of particles, including titanium dioxide particles, surface treated with dense silica-containing codeposited silicates and/or metal oxides selected from the group of silicates and oxides of metals which form insoluble silicates at a pH between five and twelve, including silicates and oxides of aluminum, tin, titanium, zinc, and zirconium.
U.S. Pat. No. 3,513,007 claims an improved process for coating titanium dioxide pigment particles comprising the treatment of titanium dioxide pigment particles in an aqueous medium, in two sequential steps, with first at least one compound selected from the group consisting of water-soluble hydrolysable compounds of silicon, titanium, zirconium, and phosphates, and secondly with at least one water-soluble hydrolysable compound of aluminum, cerium, calcium, or mixtures thereof, while maintaining the pH of the suspension in the range of six to ten. The pigments produced according to the process of the invention are said to exhibit higher tinting strength and gloss when incorporated into paints.
Great Britain Patent 1,256,421 describes an improved process for treating metal oxide particles which have already been treated with a coating of one or more oxides or hydrous oxides of titanium, aluminum, cerium, silicon, zinc, zirconium, or a phosphate, with an alkaline aqueous solution of hydrolysable aluminum salt to provide a second alumina coating. Specific examples of the initial mixed oxide coatings comprise titania/alumina or zirconia/alumina. Such treatments are said to result in improved pigment durability and gloss properties.
U.S. Pat. No. 3,649,322 discloses an aluminum silicate-encapsulated pigmentary titanium dioxide, combining high tinting strength and durability in coating compositions, which is prepared by co-precipitating hydrous silicon oxide with hydrous aluminum oxide onto titanium dioxide in aqueous slurry, to form a dense coating of aluminum silicate. When the dense aluminum silicate coating is applied in a single stage, the pigment is further treated with an additional coating of aluminum oxide.
U.S. Pat. No. 3,825,438 claims a process for coating titanium dioxide pigments with at least one hydrous oxide of a metal, comprising mixing an aqueous dispersion of titanium dioxide pigment with at least one water-soluble hydrolysable compound of a metal selected from the group consisting of aluminum, titanium, cerium, zirconium, silicon and zinc, then adding to the dispersion a polyhydric alcohol containing at least two hydroxy groups and from two to eight carbon atoms, and finally precipitating a hydrous oxide of the metal onto the surface of the particles of titanium dioxide by effecting a change in the pH of the dispersion. The examples teach co-precipitated treatments derived from mixed solutions of titanyl sulfate and aluminum sulfate. The pigments produced by the process of the invention can be used in a wide variety of products, including paints, plastics, and paper.
U.S. Pat. No. 4,052,224 discusses a process for treating a titanium dioxide pigment, using first a mixed solution of water-soluble compounds of aluminum, zirconium, and titanium, and then providing a final inorganic surface treatment with an aluminum phosphate. The resulting pigments are described as particularly useful in the manufacture of paints having reduced photochemical activity, and in the manufacture of paper laminates.
U.S. Pat. No. 4,115,144 describes the co-precipitation of metal oxides (such as, for example, alumina and titania) onto a titanium dioxide pigment, through dissolving water-soluble compounds that will precipitate as or be convertible to the desired mixed metal oxide form in water, and then adding this solution to an aqueous dispersion of titanium dioxide and precipitating the metal oxides under alkaline conditions as through the addition of sodium hydroxide. After filtration and washing, the coated and washed titanium dioxide is hot aged in the presence of water under alkaline conditions, the hot ageing step being described as necessary to avoid processing difficulties and give “suitable charge and pH characteristics”.
U.S. Pat. No. 4,328,040 describes a process for the production of titanium dioxide pigments with “improved chalking resistance and gloss retention”, wherein oxides and/or phosphates of titanium, zirconium, aluminum and silicon are applied to titanium dioxide, by adding alkaline zirconium carbonate complexes of the alkali metals or ammonium to an aqueous alkaline pigment suspension, and then adding a solution of dissolved compounds of titanium and/or aluminum and/or silicon and/or phosphorus to slowly precipitate the oxides and/or phosphates onto the pigment.
U.S. Pat. No. 4,405,376 provides a titanium dioxide pigment and a process for making the same, wherein the pigment comprises a pigmentary titanium dioxide core particle, a mixed inner coating of hydrous oxides of tin and zirconium, and an outer coating of a hydrous oxide of aluminum.
U.S. Pat. No. 4,450,012 discloses coated rutile mixed phase pigments having a first coating of an oxide or mixture of oxides of titanium, zirconium, or tin, and a subsequent coating of an oxide of aluminum. The resulting pigments exhibit an improved tendency against flocculation in lacquers hardened with acid catalysts.
U.S. Pat. No. 4,759,800 describes a process for chemically treating titanium dioxide pigment wherein titania is deposited first from a solution of titanium oxychloride, and then an alumina outer treatment is performed. Many examples illustrate the co-deposition, or co-precipitation, of other metal oxides along with the deposited titania, including titania/alumina, titania/zirconia, and zirconia/titanialsilica co-precipitated combinations. The resulting pigments purportedly exhibit improved weathering resistance and optical properties.
U.S. Pat. No. 4,781,761 discloses that co-deposition of boria with silica, preferably from a master solution containing water-soluble sodium silicate and sodium borate, enables the formation of dense silicate coatings on titanium dioxide particles at lower processing temperatures than used previously to achieve dense silica coatings. The resulting boria-modified silica-containing pigments are highly lightfast, and exhibit excellent gloss and dispersibility.
U.S. Pat. No. 5,753,025 discloses a process for making a rutile titanium dioxide pigment suitable for use in making coatings having improved gloss, through co-deposition of boric with silica, followed by treatment with an oxide of aluminum.
U.S. Pat. No. 7,135,065 describes the production of titanium dioxide pigments in which aqueous solutions of water-soluble compounds of tin and zirconium as well as at least one more of aluminum, silicon and titanium are added to an aqueous suspension of titanium dioxide base material maintained at a pH of not more than 3 or less than 10, and then the pH value of the suspension is adjusted to between 6 and 8 to cause the corresponding oxides to be deposited on the titanium dioxide base material.
U.S. Patent Application Publication 20040025749 A1 discloses a method for preparation of titanium dioxide pigment exhibiting high greying resistance and high hiding power, in which the pH value of a suspension of titanium dioxide material, a phosphorus compound, a titanium compound and an aluminum compound is adjusted to about 9, followed by the addition of a magnesium compound while maintaining the pH value above about 8.5.
U.S. Patent Application Publication 20050011408 A1 describes a method for the surface treatment of a titanium dioxide pigment, comprising the steps of: a) adding an aluminum component and a phosphorus component to a titanium dioxide suspension while the pH value of the suspension is maintained at a value greater than or equal to ten; and then b) adding an acid component to the suspension until the pH value is less than nine. It is also taught that together with the aluminum component and the phosphorus component, other metal salt solutions, such as salts of cerium, titanium, silicon, zirconium, or zinc, can also be added to the suspension in step a), these subsequently being jointly precipitated onto the particle surface in step b) as phosphate or hydrated oxide.
U.S. Patent Application Publication 20060032402 A1 relates to titanium dioxide pigment particles having two or more layers deposited thereon, wherein at least one of the two or more layers is a dense silicon dioxide layer comprising silicon dioxide containing no significant quantity of metal atoms other than silicon, and wherein at least one of the two or more layers is a dense silicon dioxide layer containing a significant quantity of co-precipitated oxides of metal ions or mixtures of metal ions other than silicon. The resulting pigments are weather resistant and particularly suitable for use in surface coatings and plastics.
U.S. Patent Application Publication 20060034739 A1 relates to a method for treatment of titanium dioxide characterized in that, together with the hydrous oxides of tin and zirconium, at least one other from the group consisting of aluminum, silicon, and titanium is additionally co-precipitated onto the particle surface. The treated pigment is subsequently treated with an oxide of aluminum. Compared with the prior art, the resulting pigments demonstrate a further improvement in photostability, while retaining good optical properties and are particularly suitable for use in paints, coatings, and plastics.
Despite the many titanium dioxide pigments thus described as having mixed inorganic oxides applied via co-precipitation or co-deposition, however, none of the aforementioned references anticipate or suggest the process efficiencies enabled by the present invention and described in greater detail below, nor the product consistency and uniformity of co-precipitant incorporation improvements realized through the present invention.